Inductive control system for railroads



H. c. KENDALL 2,817,012

5 Sheets-Sheet l F ICJJ;

GAT E GENERATOR INVENTOR.

MASTER CHANNEL 16 SATURABLE PULSE fiTRETCHER- ANPLiFlER PULSE STRETCHER-PUL5E STRETCHER- AMPUHER PULSE TRANSFORMER CONTROLLER AMPLIFIERSTRETCHER- AHPLlFiER CZP MWTOOTH 1 E ERNQR DFFERENIIAT AMPLlFlER ,25CATHODE CATHODE FOLLOWER CATHODE INDUCTIVE CONTROL SYSTEM FOR RAIL-ROADS)TRMKWAY RECEIVING COIL l0 E l H.C.. KENDALL Hi5 ATTORNEY cameo CATHODEFOLLOWER 18 AHPLlTUDE AMPLlFlER 'GATED. AHPUHER" FoLLowER AMPuHER.FOLLOWER GATED AMPuFIER.

Filed Feb. 20. 1952 TRAIN CAR REE CONTROL C0111 OSCILLATOR 5WEEP 1:0RESONANT UNiT MID DET.

BLOCKING FREQUENCY P-DETECTOR & OSQLLATOR (ACCEPT? H) Z- L-cREs0NAm UNITAND on.

. mcEPTs f2) [:CRESONANT UNIT AND DET (ACCEPTS fa] Dec. 17, 1957 H. c.KENDALL INDUCTIVE CONTROL SYSTEM FOR RAILROADS Filed Feb. 20. 1952 5Sheets-Sheet 2 IN VEN TOR.

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INDUC'I'IVE CONTROL SYSTEM FOR RAILROADS Filed Feb. 20, 1952 L 5Sheets-Sheet 5 TYPICAL WAVEFORMS VOLTAGE v QAWTOOTH GENERATOR OUTPUTFREouENoI n I/ I I B FREQUENCY SWEEP FREQUENCY OSCILLATOR OUTPUT VOLTAGEAFFECTED BY TUNED CONTROL COIL I I I I v D kVOLTAGE k V k OUTPUT OFDIFFER- I 1 I I I ENTIATOR-AMPLIFIER 19 I OUTPUT OF L-C RESONANT UNITAND DETECTOR FOR CHANNEL 1 F I m GATING VOLTAGE AT I I I I I I VOLTAGEPLATE OF TUBE 106 G OUTPUT 0F CATHODE FOLLOWER 25 FOR CHANNEL 1 Y OUTPUT0F PULSE I I I TRETQHER AMPLIFIER V0 LTA RELAY DROPPED AWAY 1 F: 7"OPERATION OF RELAY c1 RELAY PIQKE'D UP I FIG-5A. FIG.5B. raw/v area/060/L6 r/xz'a Ha/r-uP'c'a/L- INVENTOR H. C. KENDALL H15 ATTORNEY CDCDCDI-L {23353 I I I ENVELOPE 0F SWEEP FREQ" UENCY OfiCILLATOR OUTPUT UnitedStates Hugh C. Kendall, Rochester, N. Y., assignor to General RailwaySignal Company, Rochester, N. Y.

Application February 20, 1952, Serial No. 272,571

9 Claims. (Cl. 246-124) This invention relates to an improved system fortransmitting controls inductively between moving trains and fixedWayside locations.

This application is to be considered in the nature of an improvementover the control system disclosed in the patent to H. C. Kendall et al.No. 2,693,525, issued November 2, 1954; and no claim is intended to bemade herein to any subject matter disclosed in such prior application.

The principles of this invention may be applied, as a particularembodiment thereof, to the transmission of controls from moving vehiclesto selected wayside locations as, for example, when it is desired todetermine the identity of trains as they pass a particular location.This train identification system may be used in manual block signalsystems, for example, where the local operator at each signal locationmust receive information as to whether or not the block ahead is clearbefore he can allow trains to enter that block. By placing, at the exitend of such a block, the proper wayside apparatus, and on the rear caror caboose of each train the appropriate vehicle-carried apparatus, allaccording to the present invention as will be described, it becomespossible for the local operator to be informed as to when each train hasfully left the block ahead.

In a train identity system of this kind, it is generally only requiredto be known that a train has passed a given location; information as tothe particular type of train as, for example, its number designation ordestination is generally not needed. However, in train describer systemswhere the route to be set up for a train is dependent upon the kind oftrain or its destination and also in train annunciator systems wheretrain designations must be determined as well as the fact that the trainhas passed a certain location, additional means must be provided whichcontrols the wayside apparatus so as to give distinctive outputs astrains of different classes or designations pass by.

The principles of this invention may also be applied, as anotherembodiment thereof, to a system for transmitting controls from thetrackway to passing trains as, for example, in an intermittent traincontrol system. In this disclosure, however, the features of the presentinvention will be shown and described more particularly in connectionwith a train identity system.

The train identity system of this invention comprises wayside apparatuswhich includes an electronic oscillator organized so as to have itsoutput sweep rapidly over a selected frequency range. The oscillatoroutput is applied to a receiving or pick-up coil which is mountedadjacent the trackway in such a manner that it can become inductivelycoupled to a tuned coil mounted upon the vehicle.

atent C F During the interval that the wayside pick-up coil and thetuned vehicle-carried coil are inductively coupled, the oscillatoroutput sweeps over its entire range a plurality of times. Thevehicle-carried control coil is tuned to a frequency Within the sweepfrequency range of the oscil- 2,817,012 Patented Dec. 17, 1957 lator sothat each time the oscillator frequency sweeps through the resonantfrequency of the tuned inductor, an energy transfer takes place from thepick-up coil to the tuned control coil. The wayside apparatus is soorganized as to detect the resultant loading effect upon the oscillator,thereby ascertaining that an inductor, tuned to a frequency within therange of frequencies swept over by the oscillator, has passed thewayside location.

Where additional information as to train designation is to be made knownas each train passes the wayside location, all the trains with the samedesignation carry one or more resonated coils each of which is tuned toa particular preselected one of a plurality of resonance frequencieswhich are included within the range of frequencies swept over by theoscillator included in the wayside apparatus. The oscillator output isthus afiected each time that its frequency sweeps over the particularresonant frequency of the vehicle carried tuned coil with which it isinductively coupled. When the train identity system of this invention isto be used in such a system, the wayside equipment is organized todetect at what particular frequency of the oscillator each reactionoccurs and so provides distinctive outputs for each of the differentlytuned vehicle-carried inductors so as to properly distinguish betweenvarious types of trains as they pass the wayside coil location.

An object of this invention is to provide a train identity system havingwayside equipment which is so organized as to be unresponsive tospurious effects which might tend to affect the oscillator in a mannersimilar to that produced by a resonated control coil.

A further object of this invention is to provide wayside apparatusincluded in a train identity system having an electron tube oscillatorcontrolled to sweep over a selected frequency range in the samedirection for successive sweeps, to thereby permit a finer tuning ofcertain tuned circuits which are included in the wayside apparatus witha resultant increase in ability to discriminate between the varioustuned vehicle-carried inductors that may be used.

Further objects, purposes, and characteristic features of this inventionwill in part be obvious from the accompanying drawings and in partpointed out as the description of the invention progresses.

In discussing this invention in detail, reference will be made to theaccompanying drawings in which like reference characters designatecorresponding parts throughout the several views, and in which:

Fig. 1 is a block diagram diagrammatically illustrating the circuitorganization of this invention;

Figs. 2A, 2B, and 2C, when placed in order, one above the other,comprise a circuit diagram showing in detail the various parts andcircuits of this invention;

Fig. 3 illustrates graphically certain waveforms as an aid in describingthe manner of operation of an embodiment of this invention,

Fig. 4 illustrates a modified form of master channel to be used incertain circumstances; and

Figs. 5A and 5B show how a plurality of coils may be provided to give acoded form of control.

To simplify the illustration and facilitate in the explanation, thevarious parts and circuits constituting the embodiment of the inventionare shown diagrammatically and certain conventional illustrations havebeen employed. The drawings have been made to make it easy to understandthe principles and manner of operation rather than to illustrate thespecific construction and arrangement of parts that would be used inpractice. The various relays and their contacts, for example, are shownin a conventional manner, and symbols are used to indicate connectionsto the terminals of batteries or other sources of current instead ofshowing all of the wiring connections to these terminals. The symbolsand indicate connections to the opposite terminals of a source ofsuitably low voltage as is required for the operation of relays and thelike. The symbol (B+) and the symbol for a ground connection indicateconnections to the opposite terminals of a source of somewhat highervoltage such as is required for the operation of various electron tubes.

Without attempting at this time to discuss in detail the scope of theinvention but merely to illustrate the general principles, Fig. 1 showsa receiving coil which is preferably mounted adjacent the trackway whenthe system is to be used for train identification purposes. Thevehicle-carried control coil 11 which is resonated by an associatedcapacitor 12 assumed to be so mounted upon a vehicle that it will,during motion of the vehicle, become inductively coupled with thewayside coil.

The wayside equipment shown in Fig. 1 includes apparatus such as theblocking oscillator 13, cathode follower 14, sawtooth oscillator 15, andsaturable transformer controller 16, all of which cooperate to cause thesweep frequency oscillator 17 to rapidly vary its output fre quency overa selected range. When the output frequency of this oscillator 17 sweepsover the resonant frequency of a control coil 11 that is inductivelycoupled to the receiving coil 10, there is a reaction on the oscillator17 which is detected by the amplitude detector and amplifier 18 and socauses an output pulse to be supplied by the differentiator-amplifier 19to the pulse stretcheramplifier 20 and to the gated amplifier 21 of eachchannel. The pulse stretcher-amplifier 20 of the master channel respondsto each such pulse and, in turn,gove'rns the operation of relay MR.Thus, the moving of a vehicle carrying a resonated control coil 11 pastthe wayside coil 10 is effective to cause actuation of relay MR of themaster channel, regardless of the particular resonant frequency of suchcontrol coil.

In addition to the master channel, there are a plurality of individualchannels, one for each of the different control coil resonancefrequencies that may be used in a given system. Each of these channelscomprises resonated circuit elements included in an L-C resonant unitand detector 22, having the output of the sweep frequency oscillator 17applied thereto as indicated by the connection over wire 23 from theoscillator 17 to each of these L-C resonant units and detectors 22. TheL-C resonant unit and detector 22 responds to this input by causing adistinctive voltage to be produced each time that the output frequencyof the oscillator sweeps over the frequency associated with thatchannel. Thus, it may be said that each of the individual channelsincludes sweep position-determining apparatus which, in effect,demarcates the time interval throughout which the output frequency ofthe sweep oscillator 17 is passing through the resonant frequency of acorresponding control coil by supplying a distinctive output voltageduring such interval.

The gated amplifier 21 of each channel has applied to it over wire 24the output pulses obtained from the differentiator-amplifier 19, thesepulses being indicative of reactions produced on the sweep frequencyoscillator 17 by resonated control coils. These output pulses suppliedby the ditferentiator-amplifier 19 are all alike, regardless of theresonant frequency of the control coil which has caused them to occur.However, the output pulse produced by the differentiator-amplifier 19 inresponse to a control coil tuned to a particular one of the plurality ofresonance frequencies can, of course, only occur at the time that theoutput frequency of the oscillator 17 is passing through this particularresonant frequency. The particular channel corresponding to thisfrequency always demarcates, as described, the interval during which theoscillator frequency is sweeping through the resonant frequency of theassociated control coil, and the distinctive voltage that is produced isapplied as a gating voltage to the gated amplifier 21 of that channel.The occurrence of a pulse from the'difierentiator-ampliiier 19 duringthe time existence of such gating voltage is an indication that acontrol coil 11 resonated to the frequency associated with that channelis then inductively coupled to the receiving coil 10. The gatedamplifier 21 is, accordingly, so organized that it produces an output inthe event that a pulse is applied to it over wire 24 at the same timethat a distinctive gating voltage is applied to this particular gatedamplifier.

The output pulses produced by the gated amplifier 21 of any channel aresupplied to an associated cathode follower 25 which then supplies acorresponding output to a respective pulse stretcher-amplifier 26. Thisdevice, in turn, controls an electromagnetic relay C1, C2, or C3associated with the channels l-3 inclusive. The actuation of a relay ofone of the individual channels along with the actuation of the masterrelay MR controls associated circuit means to a distinctive condition,thereby providing an indication as to the type or designation of trainthat has passed the wayside location.

In the embodiment of the invention illustrated in block diagram form inFig. 1 and also in detailed circuit form in Figs. 2A, 2B, and 2C, threeindividual channels have been shown in addition to the master channel soas to accommodate three different train identities. More or lesschannels may, of course, be used as desired.

With respect to the apparatus which is effective to controlthe'operation of the sweep frequency oscillator 17, Fig. 1 shows thatthe blocking oscillator 13 supplies its output pulses which occur at apreselected frequency to a cathode follower 14 which then applies thesepulses to the sawtooth oscillator 15. It should be understood thatvarious types of frequency sources could well be used in place of ablocking oscillator, and, if desired, the sawtooth oscillator 15, in amanner well-known in the art, may be so organized as to operate at itsown natural frequency as determined by the values of its circuitcomponents.

The output of the sawtooth oscillator 15 comprises a voltage waveformwhich varies in a somewhat linear fashion between a lower and a higherlimit with the return to its origin occurring in a very abrupt and rapidmanner as is approximately illustrated in Fig. 3 at line A. This voltagewaveform is applied to a circuit organization termed a saturabletransformer controller 16. This device acts to vary the current throughthe primary winding of a saturable transformer to thereby cause avariationin the inductance of its secondary winding. The saturabletransformer controller 16 cooperates with the sweep frequency oscillator17 by causing a variation in the inductance of the tuned circuitsincluded in the sweep frequency oscillator, thereby causing the outputfrequency of this oscillator to vary rapidly over a selected frequencyrange as at line B of Fig. 3. Thus, the blocking oscillator 13determines the sweep repetition rate at which the output voltage of thesawtooth oscillator 15 varies between its lower and higher limits andfor each such voltage variation in the output of the sawtooth oscillator15, the inductance of the secondary winding of the saturable transformeris caused to vary and thus vary the oscillator frequency.

The gate generator 27 is also controlled by the output of the sawtoothoscillator 15 and allows the differen'tiatoramplifier 19 to respond onlyfor a limited time, as will presently be more fully described, so as toprevent its responding to transient voltage variations.

The trackway receiving coil 10 is actually included in the circuitorganization of the sweep frequency oscillator 17 so that the couplingof a vehicle-carried control coil 11 to the wayside coil 10 causes aloading of the oscillator as the oscillator frequency sweeps through theresonant frequency of the tuned control coil. This loading efiect causesthe envelope of the oscillator output to decrease and then to abruptlyincrease again when the coupling ceases as shown at line C of Fig. 3.The amplitude detector and amplifier 18 rectifies the output of thesweep frequency oscillator 17 and filters out the high frequencyvariations so that the output of this amplitude detector and amplifierincludes substantially only the waveform of the envelope of theoscillator output as shown in Fig. 3 at line C. Theditferentiator-amplifier 19 responds to the sudden increase in amplitudeof the input applied to it at the time the loading efiect ceases bysupplying an output pulse (see line D, Fig. 3) over wire 24 to thevarious gated amplifiers 21, each associated with a respective channeland also to the pulse stretcher-amplifier 20 of the master channel.

Since the pulse stretcher-amplifier 20 of the master channel has appliedto it the output from the differentiator-amplifier 19, it receives aninput pulse each time that the frequency of oscillator 17 sweeps overthe resonant frequency of a control coil coupled inductively to thereceiving coil. During the time that the receiving coil and control coilare inductively coupled, the oscillator frequency sweeps through theresonant frequency of the vehicle-carried inductor a plurality of timesso that a succession of output pulses is provided by thedifierentiatoramplifier, one for each such sweep. The rate of occurrenceof these outputs is determined by the rate of frequency sweeping andthus is determined, in effect, by the repetition rate of the blockingoscillator 13 output. The pulse-amplifier 20 responds to each pulsereceived from the difierentiator-amplifier 19 by producing an outputwaveform which is at a high level for approximately half of the timebetween successive outputs and at a reduced level for the other half ofeach interval as shown at line H of Fig. 3. Thus, an essentiallyrectangular waveform with successive half-cycles of approximately equallength is supplied to the associated full-wave rectifier 28, and therectified current supplied by the output of this rectifier is utilizedto energize the relay MR. Consequently, the picking up of the armatureof relay MR is an indication that a control coil 11 has becomeinductively coupled to the receiving coil 10.

Relay MR associated with the master channel is provided with slowoperating characteristics by reason of the capacitor 29 connected acrossits winding. As a result, a plurality of successive outputs from thedifferentiatoramplifier 19 must occur in order to allow the rectifiedoutput from the full-wave rectifier to persist for a long enough time toallow relay MR to pick up. In this way, random, spurious outputs whichmight appear in the output of the ditferentiator-amplifier 19 are noteffective to cause actuation of relay MR.

The detailed circuit organization of one embodiment of this invention isshown in Figs. 2A-2C. The dotted line blocks in this drawing have beenmade to correspond in designations with the blocks of the diagram ofFig. 1.

As previously stated, various types of frequency generators may be usedto determine the sweep repetition rate of the oscillator 17 even thoughthe specific form shown in Fig. 2A comprises a blocking oscillator. Thisblocking oscillator 13 includes a tube 40 which has its plate connectedthrough a winding of a pulse transformer T1 and resistor 41 in series to(3+). The control grid is connected through another Winding of the pulsetransformer T1 and through resistor 42 and capacitor 43 in parallel toground. A by-pass capacitor 44 is connected from the junction ofresistor 41 and the plate winding of the pulse transformer T1 to ground,and another connection is made from ground, and through a third windingof the pulse transformer T1 to the control grid of tube 45 included inthe cathode follower 14. The control grid of tube 45 is connected toground through resistor 9 which thus effectively shunts this outputwinding of transformer T1 and thereby dampens transient voltages thattend to appear across this Winding. Thus, a conventional blockingoscillator is provided which supplies positive output pulses to thecontrol grid of the cathode follower tube 45 at a rate which isprimarily determined by the time constant of the parallel-connectedresistor 42 and 6 capacitor 43. Since a blocking oscillator of this kindis well-known in the art, its operation will not be described in detail.

Each positive input pulse applied to the control grid of the cathodefollower tube 45 causes a corresponding positive pulse to appear acrossthe cathode load resistor 46 of this tube, and these positive pulses areapplied through a capacitor 47 to the control grid of a triode tube 48which is included in the sawtooth generator 15. The occurrence of apositive pulse on the grid of tube 48 causes a current to flow betweenthe cathode and control grid of this tube so as to charge the capacitor47. At the end of this positive pulse, the charged capacitor 47 beginsto discharge but can now discharge only through the relatively highresistance of resistor 39 which connects the control grid to (13+). Forthe selected rate at which the positive input pulses occur, asdetermined by the blocking oscillator 13, the time constant for thedischarge circuit of capacitor 47 is so selected that this capacitor candischarge only slowly between successive input pulses. Consequently, thecontrol grid of tube 48 is maintained at a sufiiciently negativepotential between successive input pulses to maintain this tube cut off.Upon each occurrence of a positive input pulse, however, the tubeconducts momentarily to charge capacitor 47.

Each time that tube 48 becomes conductive in response to a positiveinput pulse, the capacitor 49 which is connected from its plate toground and is normally charged to a relatively high voltage after thetube has been in a cutoff condition for some time, discharges throughthe plate-cathode circuit of conducting tube 48. This discharge takesplace rapidly so that the plate voltage of tube 48 abruptly drops to arelatively low value.

A connection is provided from the plate of tube 48, through capacitor50, to the control grid of tube 51 included in the gate generator 27.The control grid of tube 51 is normally at about ground potential sinceany increase in potential of this grid above ground causes a gridcurrent to flow through the resistor 52 connecting this control grid to(B+) which tends to lower the grid voltage. This tube 51 is normally,therefore, in a conductive condition, and the capacitor 50 is normallycharged to a relatively high voltage.

As the plate voltage of tube 48 is lowered as a result of its becomingconductive and allowing capacitor 49 to discharge, the capacitor 50 alsotends to discharge, but since the discharge can take place only throughthe relatively high resistance of resistor 52, this capacitor 50 candischarge only slowly with the result that the grid voltage of tube 51is driven negative with respect to its cathode and is cut ofi.

The cathode follower comprising tube 53 is associated with the tube 48so as to provide means for increasing the linearity of the sweep outputvoltage obtained from tube 48. More specifically, by choosing theseries-connected cathode load resistors 54 and 55 of this tube to be ofa large value, the cathode voltage of tube 53 approximately equals thegrid input voltage to this tube which is obtained from the plate of tube48. Since this cathode voltage is applied through a capacitor 56 to thejunction of resistors 57 and 58 which connect the plate of tube 48 to(B+), a substantially constant voltage difference appears across theresistor 57 so that the charging current for capacitor 49 is maintainedat an essentially constant value. The voltage across the chargingcapacitor 49 thus increases at a substantially linear rate with respectto time.

When capacitor 49 begins to charge through resistors 57 and 58 aftertube 48 has again become nonconductive, no charging current need atfirst be supplied for the capacitor 50 since the voltage existing atthat instant between the plate of tube 48 and ground is less than thevoltage then appearing across the capacitor 50 so that this capacitor 50continues to discharge for a short time. The reason for this occurrenceis that the slow discharge rate of the capacitor 50 prevents it fromdischargingfully during the short interval of time that tube 48 conductsin response to thepositive output pulse obtained from the cathodefollower 14. Therefore, at the ins'tanttube 48 becomes nonconductiveonce more, the capacitor 50 continues for a'short time to dischargestill more. After capacitor 49 has charged for a short time, however,the voltage at the plate of tube-48 rises to a levelwhere it equals thethen existing voltage across the capacitor 50. Any further increase ofvoltage at the plate of'tube'48 must now cause an increase in the chargeon the capacitor 50. The effect of this occurrence is to cause thevoltage at the plate of tiibe48 to rise more rapidly at first and thento 'increaseat' a somewhat slower rate during "the remainder of thevoltage'rise when charging current must also ;be supplied" forthe'capacitor 50 as'well as for the capacitor'49. As a result, 'thevoltage that appears at the junction of resistors '54- and-55 in thecathode circuit 'of tube"53 has approximately the waveform shown in Fig.3 atline A.

The saturable transformer controller 16 includes the pentode type tube59 which has its cathode connected directly to ground and its controlgrid connected through a grid leak resistor 60 to ground. 'Theiriputvoltage is applied through the coupling c'a'p'a'citor97 "to the controlgrid. The screen grid is connected to the plate, and the suppressor gridis connected to the cathode. The primary winding of the saturabletransformer ST is included in the plate circuit of this tube in serieswith a resistor 61. The resistor 62 connected across the primary windingof the saturable transformer ST serves to dampen transient voltageswhich tend to appear in the primary circuit. It has been found that theQ of the secondary winding of transformer ST decreases considerably whenthere is no current in the primary winding and this would ordinarilytend to make the oscillator 17 momentarily inoperative at the end ofeach frequency sweep when the current through tube 59 drops to zero.Resistor 38 prevents this from happening by maintaining a certainminimum current through the primary'winding.

The outstanding characteristics of the saturable transformer ST thatmake possible its use in conjunction with a radio-frequency sweeposcillator organization are its very low secondary inductance and thevery low degree of coupling between primary and secondary windings whichminimize the shunting effect on the sweep frequency oscillator 17 Byapplying an input voltage to the control grid of this tube 59 having awaveform substantially as shown in Fig. 3 at line A, the variation incurrent through the primary winding of the saturable transformer isapproximately linear with respect to time. The high inductance appearingin the plate circuit of tube 59 through the effect of the primarywinding of transformer ST would ordinarily tend to prevent a rise ofplate current at the start of each sweep; however, the more rapidincrease of voltage occurring at the start of the upward voltage sweeptends to overcome this effect so that the plate current of tube 59varies almost linearly. As will presently be more fully described, thesecondary winding of this saturable transformer ST is included in thecircuit organization of an electron tube sweep oscillator 17. Thevariation in inductance of the secondary winding serves to vary thefrequency of the oscillator output.

It has been found that a substantially linear current variation in theprimary winding of the saturable transformer ST tends to produce aninductance variation of 'the secondary winding effective to produce analmost linear thus also a more linear frequency variation with time canbe achieved. It should be fully understood,'however, that although thecircuit organization 'as' shown in Fig. 2 tends tolproduce an-equencvariation that is substantially linear with time, the principles of thisinvention do not 'requirethat the frequency sweeping be accomplished ata linear rate as it is only required that the frequencys'weepin'g'thr'ough aselected range be at a relativelyrapid andsomewhatunirorm rate.

The voltage applied tothe control grid of tube 59 is shown at line A ofFig. as increasing from-a low value to some higher level and'th'endecreasing abruptly to its original value. This 's'tidden voltageflyback of voltage tends to produce aspariens output from the secondaryof the saturable transformer ST and may be effective throughout the restof the circuit organization 'to provide an output similar to that whichoccurs when a tuned control coil is brought into coupling range with thereceiving coil. Aswill later be'apparent, anoutput from the waysideequipment caused by such-spur-iousyoltages can be prevented "fromoccurring by removing or sufficiently lowering 'the'pla'te voltage oftube 63 included in the diiTerentiato'r-amplifier 19. For this reason,the output voltage appearing at the plate of 'tube 48 included in thesawtooth oscillator 15 is supplied, as previously described, to thecontrol grid "of the normally conductive tube 51 which is included inthe gate generator 27. As a result, upon the occurrenceof the abruptvoltage drop at the plate of tube 48, tube '51 becomes momentarilycutoff as'previously described and the plate voltage of this tube'51 is"thus momentarily increased because of the reduced voltage drop'a'crossresistor 64. This voltage increase'is applied through capacitor 65 tothe control grid of'tube'65 and causes this latter tube to becomemomentarily conductive. The resulting reduction in the plate"voltageo'f"this tube -causes a sufficient reduction in the plate voltage 'oftube63 included in the differentiator-amplifi'er19tomake this tubeinoperative for a time.

The positive plate 'pulse'of"tube 51'also causes the capacitor "*65 toquickly charge, and since the time constant for the discharge of thiscapacitor through the resistor 98 is chosentobe relatively long withrespect to the interval between successive inputs, a negative cutoffvoltage 'is'applied to 'th'e control grid of tube 66' between successivepulses. The high plate voltage that then results for tube 66 because "of'the reduced voltage drop across plateresistor 99 causes "the normallyhigh operating potential to be supplied to the plate of tube 63 for theremainder of each sweep cycle. Only at the beginning of each sweep cyclewhen "the abrupt voltage reversal occurs does the tube'66 becomemomentarily conductive and so disable tube 63 by reducing its platevoltage.

The sweep frequency oscillator 17 shown in Fig. 2A is relatedto theColpitts type'of oscillator circuit, but other types of oscillatorcircuits may also be'used when desired. In this'oscillator, thegrid-cathode circuit includes a parallel inductance-capacitance tankcircuit which includes the secondary winding of the saturabletransformer ST shown iu'the saturable transformer controller 16.Thissecondary winding is connected in parallel with the fixed inductor'67. The capacitance elements of this tuned circuit'comprise thecapacitors 63 and 69 connected in series,'withthe cathode of the tube 70connected to their junction. The cathode circuit of this tube alsoincludesthe wayside receiving coil '10 so that this wayside coil isactually in parallel with the capacitor 69 and thus forms a .part of thetuncd circuit for the oscillator. The voltage appearing across thistuned circuit is coupled "through the coupling capacitor 71"to thecontrol grid of the tube 70, and this control grid -is connected toground through the grid leak resistor 72. The plate of the oscillatortube 70 is connected through the plate load resistor 73 to (13+),a'ndthis resistor is shunted for the range of output" frequencies of-theoscillator by the by-pass capacitor 74 which is -conne'cted from plateto ground. -The screen' grid of this tube 70 is connected directly tothe plate, and the suppressor grid is connected to the cathode.

The output of the sweep frequency oscillator 17 is obtained from itscathode and applied directly to the control grid of tube 75 included inthe cathode follower-amplitude detector 18 which also includes the tube76. The cathode circuit of tube 75 includes the resistor 77 which isshunted by the capacitor 78. On the positive peaks of the input voltageto the control grid of tube 75, the cathode-plate current reaches itsmaximum value, and the maximum voltage then appears across the cathoderesistor 77. The capacitor 78 tends to charge to a voltage levelapproximating this maximum voltage that appears across the cathoderesistor. The time constant for the capacitor 78 and resistor 77 is sochosen that the capacitor 78 can discharge only slightly betweensuccessive positive halfcycles so that a relatively high level of directvoltage is maintained between the cathode of tube 75 and ground. Thisbias voltage is of a sufficient level to cut the tube off for thenegative half cycles of the input voltage with the result thatrectification of the input to this tube occurs.

The time constant for capacitor 78 and resistor 77 is properly chosen,however, to permit variations in the cathode voltage of tube 75 whenchanges occur in the level of the output voltage derived from the sweepfrequency oscillator 17. In other words, this time constant is chosen tobe sufficiently long so as to prevent a substantial discharge ofcapacitor 78 during the interval be tween successive cycles of theoscillator output, but is sufliciently short so as to permit dischargingof this capacitor in the event, for example, that a decrease in outputvoltage of the oscillator occurs.

A resistor 79 and the capacitor 80 which are connected in series acrossthe cathode load resistor 77 act as a low pass filter for the ouput ofthe amplitude tube 75. Thus, the capacitor 80 acts as a low impedancefor the relatively high frequency output of the oscillator, but theslower variations in amplitude of the oscillator waveform are notshunted by this capacitor so that they are applied through a couplingcapacitor 81 to the control grid of the amplifier tube 76. This tube isprovided with a grid leak resistor 120 and plate load resistor 121 andoperates as a conventional voltage amplifier.

The principal effect of the loading of the sweep frequency oscillator 17that occurs when the receiving coil becomes inductively coupled with thetuned control coil 11 is a substantial reduction in the amplitude of theoscillator output. This drop in voltage may be compared to the decreaseof output voltage of a generator that occurs when a heavy load is placedupon it. Thus, the presence of a resonated coil in the presence of thereceiving coil which is included in the oscillator circuit causes asubstantial transfer of energy to take place from the oscillator to thetuned coil with the result that a decrease of oscillator output voltageappearing at the cathode of tube 70 takes place.

Another effect of the loading action that has been observed is atendency for the tuned control coil to momentarily lock the oscillatorfrequency to its own resonant frequency even though the oscillatorfrequency tends to continue its frequency sweeping action. In otherwords, the effects of the oscillator loading are so pronounced as theoscillator frequency passes through the resonant frequency of thecontrol coil as to tend to retard for a very brief instant the normalvariation in frequency of the oscillator that would otherwise occur.

As the inductance of the secondary winding of the saturable transformerST is varied still further by the output of the sawtooth oscillator 15,the circuit constants for the sweep frequency oscillator 17 areeventually so altered that it can no longer oscillate at a differentfrequency than the frequency at which it would normally oscillatedespite the effect of the resonated vehicle-carried coil. At such time,the oscillator output effectively becomes unlocked from the effects ofthe resonated coil, and at such time there is a rather abrupt jump inthe level of the oscillator output; Observations show that this voltagevariation is more abrupt than the decrease of oscillator outputoccurring as the resonated coil and wayside coil first become coupledtogether. The effect on the envelope of the oscillator output isapproximately as illustrated in Fig. 3 at line C and this is the voltagevariation that is applied to the control grid of tube 25' included inthe amplitude detector and amplifier 18.

The dilferentiator-amplifier 19 is organized to detect this sudden shiftin level of the oscillator output. Tube 76 included in the amplitudedetector and amplifier 18 amplifies and inverts the waveform of theoutput of tube 75, and the output voltage of this tube 76 is applied tothe control grid of tube 63 through capacitor 82 and resistor 83. Theinversion of the waveform produced by tube 76 causes the abrupt increaseof the oscillator output as the loading effect on the oscillator isremoved to appear as a sharp decrease of voltage on the grid of tube 63.

The control grid of triode tube 63 is connected to (B+) through thecurrent limiting series-connected resistors 83 and 84. The voltage atthe control grid of this tube is, therefore, normally at approximatelythe same level as the cathode. Thus, the increase of grid voltageoccuring when the oscillator first is loaded by the tunedvehicle-carried coil can only produce a further increase in the gridcurrent of tube 63 so that the plate current of this tube remainssubstantially unchanged.

The time constant for the discharge of capacitor 82 is chosen to berelatively short so that the input circuit for tube 63 is effective toproduce differentiation of the input waveform. Thus, when the platevoltage of tube 76 is abruptly decreased each time that the couplingeffect on the oscillator is removed, a negative voltage pulse appears onthe grid of tube 63 and causes this tube to be momentarily cut off.Since the junction of resistors 83 and 84 is maintained at a certainvoltage level above that of the control grid, the pulse from the plateof tube 76 must lower the voltage at the junction of these resistorsmust be lowered by more than this amount before there is any decrease inthe grid voltage of tube 76. This provision tends to prevent theoccurrence of an output from tube 76 in response to spurious voltages ofa lower level.

The positive trigger pulse that appears on the plate of tube 76 isapplied through a resistor 85 to the control grid of a cathode followertube 86. The cathode of this tube 86 is connected through resistor 122to (B+). Consequently, even when the gate generator 27 causes a normalplate voltage to be applied to tube 63 so that a higher voltage resultson the grid of tube 86, the positive cathode voltage of tube 86 cutsthis tube off so that only a substantial positive pulse at the plate oftube 63 can make tube 86 conduct. A corresponding positive pulse thenappears across the cathode resistor 87 for this tube 86 and is appliedover wire 24 to the gated amplifier 21 provided for each of the variouschannels and also to the pulse stretcher-amplifier 20 of the masterchannel.

Where a single tuned coil is provided on each vehicle, the outputsobtained from the cathode follower tube 84 include only one pulse foreach sweep of the oscillator frequency range. Since the rate of thisfrequency sweeping is determined by the blocking oscillator 13, the rateof occurrence of successive outputs from the cathode follower tube 86 isknown.

The pulse stretcher-amplifier 20 of the master channel includes thetriode tube 88 which has applied to its control grid the positive outputpulses obtained from the cathode follower tube 86. Each positive pulseapplied through capacitor 89 to the control grid causes the tube tobecome momentarily conductive and at the same time causes the capacitor89 to become charged. At the end of each such positive pulse, thecapacitor 89 is PIC;- vented from discharging rapidly because it canthen discharge only through the high resistance provided by the resistor90 connected from control grid to (B+).

The tube 88 is cut otf,therefore, following each positive pulse appliedto its grid for a time which'is determined by the discharge time of thecapacitor '89. The time constant for the discharge of this capacitor isproperly chosen so as to hold the tube 88 cut off for an interval whichis approximately equal to one-half the time between successive positiveinput pulses. The tube 88, therefore, is alternately cut off and thenconductive for approximately equal time intervals as long as "successiveinput pulses are applied to 'it from the cathode follower tube 86.

The resulting pulses "at the plate of tube 88 are then applied throughthe capacitor 91 to the control grid of amplifier tube 93. Thecontrolgrid of this tube -is connected through a grid leak resistor 94to ground, and a cathode bias is provided 'for it by the -resistor95which is shunted by the by-pass capacitor 96. The primary winding of atransformer T2 is included in the plate circuit, and the secondarywinding of this transformer supplies an output to the full-waverectifier 28 which has a relay MR connected across its output terminals.The amplified alternating input applied to tube 93 and appearing in itsplate circuit is thus rectified so as to provide a direct current forthe energization of relay MR.

This relay MR and the corresponding relays provided for the variouschannels are all provided with slow operating characteristics which maybe provided by-a shunting capacitor as previously'described. Other meansmay also be supplied as desired to require a repetition of outputs fromeach channel before the relay for that channel is actuated. Thus, itmight be required that a certain number of output pulses as determinedby a counting means be supplied to each relay before the relay could bepicked up.

Each of the various channels such as channel 1 shown in Fig. 2B includesan L-C resonant unit and detector 22 comprising a parallel tuned circuitresonated to a corresponding one of the plurality of control coilresonance frequencies. The frequency band for each of these tunedcircuits is chosen so as to encompass one of the resonance frequenciesof a vehicle-carried control coil. The tuned circuit for the resonantunit and detector 22 of channel 1 is included in the grid-cathodecircuit of tube 100 and comprises the inductor 101'and capacitor 102connected in parallel. The grid input voltage for the tube 100 isobtained from the grid circuit of the sweep frequency oscillator tube 70over the wire 23. This input voltage is applied through a decouplingresistor 103 to the control grid of tube 100.

Each time that theoutput of thesweep frequency oscillator 17 passes overthe frequencyfor which the inductor 101 and capacitor 102 exhibitresonance characteristics, the voltage on the grid of tube 100 increasesto a maximum value as the frequency approaches the exact resonancefrequency and'then decreases as the frequency sweep continues.The-plate-cathode current of this tube 100 varies-in'accordancewith'this variation in grid voltage.

The circuit organization, including the cathode resistor 104 and theshunting capacitor 105 associated with tube 100 operates in a mannersomewhat similar to that of the amplitude detector and amplifier 18 inthat rectification of the input voltage to the tube 100 occurs. Thisoutput voltage does not include any abrupt variations as does theenvelope of the sweep frequency oscillator output when this oscillatoris inductively loaded by a tuned control coil. The reason for this isthat the grid of the oscillator is sufficiently overdriven so that theloading that occurs does not greatly affect the alternating voltage onthe grid although there is a substantial decrease of the alternatingcathode voltage applied to tube 75. It is, therefore, possible to employa capacitor 105 in the cathode circuit of tube 100 having suflicientcapacitance to provide a filteringation for the oscillator frequencys'othatfihebutputvoltage obtained from the cathode of this tuberepresents approximately the envelope of the input grid voltagewaveform. Additional filtering means such as is provided for the outputof tube included in the amplitude detector and amplifier by resistor 79and-capacitor 80 is not required.

The output voltage of tube is applied to the cathode of tube 106included in the gated amplifier 21. T he'control grid of this tube ispositively biased by having'its grid connected to the junction ofresistors 107 and 108 which are included between (3+) and ground.Accordingly, when the amplitude of voltage obtained from the cathode oftube 100 rises above a predetermined level, the tube 106 becomes cutoif. Variation of the positive grid voltage for the tube 106 by changingthe ratio of values of the voltage dividing resistors 107 and 108aifects the amplitude of voltage that must be applied to its cathode inorder to cut the tube off. Since the amplitude of the applied cathodevoltage varies in accordance with the proximity of the sweep frequencyvoltage to the exact resonance frequency of the tuned circuit in thegrid circuit of tube 100, the grid bias on tube 106 can be adjusted soas to cut off this tube for a selected range of frequencies centeringabout the frequency F1.

For example, if the resonant frequency to which a vehicle-carried coilmay be tuned is 280 kc., then the inductor 101 and capacitor 102 aretuned to this frequency also. As the output of the swee frequencyoscillator 17 approaches 280 kc., and then decreasing again as thefrequency recedes from this value. The cathode output voltage of thistube varies in a similar manner. Since the selection of bias voltage fortube 106 controls the level of cathode voltage that must be applied tocut this tube off, this also determines the range of output frequenciesof the sweep frequency oscillator 17 for which the tube is cut oif.

Since tube 106 is normally conducting because of its positive gridvoltage, the voltage at the plate of this tube is normally at a lowvalue, so that the voltage applied to the control grid of the cathodefollower tube 110 is low. Since the cathode of tube 110 is positivelybiased by being connected through resistor 92 to (B+), the low gridvoltage results in a grid-cathode voltage for this tube that is belowcutoff.

Each time that the output of the sweep frequency oscillator 17 sweepsthrough the frequency F1, however, tube 106 becomes cut off as has beendescribed. The voltage increase that then tends to appear between plateand cathode of this tube 106 causes tube 109 to conduct a substantialplate current because of the normally positive bias for this tuberesulting from the connection of its control grid to the junction ofresistors 118 and 119 connected between ground and (13+). The flow ofplate current of tube 109 through the resistors 113 and 114 produces asubstantial voltage drop so that a low voltage still appears on the wire111 which is connected to the grid of cathode follower tube 110.However, if a vehicle-carried coil tuned to F1 is at'that time coupledto the wayside receiving coil 10, the loading of the swee frequencyoscillator 17 that-occurs causes a positive output pulse to appear onthe wire 24 in a manner that has already been described in detail. Thispositive pulse is applied through the capacitor 112 to the cathode oftube 109 and raises the cathode voltage appearing across resistor 122 tosuch an extent that the tube is momentarily cut 01f. The drop in currentthat then momentarily results through the resistors 113 and 114 causes apositive pulse to be applied to the control grid of tube 110. In otherwords, if at the time that the oscillator output is sweeping over therange of frequencies adjacent F1, an output is simultaneously receivedfrom the difierentiator-amplifier 19 resulting from aloading action uponthe oscillator, this is an indication that the reaction which has beendetected-is caused by a coil tuned to F1, and therefore a positive pulseis applied to "the cathode follower tube1 10 bf-channel 1.

During the interval that the control and receiving coils are inductivelycoupled together, a distinctive output pulse is obtained from thecathode follower 25 across the oathode resistor 115 for each sweep ofthe oscillator frequency through the frequency F1. Since the rate offrequency sweeping is dependent upon the relatively fixed outputfrequency of the blocking oscillator 13, the consecutive output pulsesfrom the cathode follower 25 occur at a predetermined frequency. Thepulse stretcher-amplifier 26 for channel 1 and for the other channelsalso is organized in a manner similar to that of the pulsestretcheramplifier 20 of the master channel previously described. Thus,in response to each output pulse from the cathode follower 25, the tube116 is cut off for approximately onehalf of the interval betweenconsecutive pulses and is then conductive for the rest of this intervalso that a rectangularly shaped waveform having positive and negativeportions of approximately equal duration is applied to the control gridof tube 117. The resulting variations in plate current are applied,through the action of the transformer T3, to a full-wave rectifier 30and are effective to cause the energization of the relay C1. This relayC1 is also provided with slow action characteristics so that a pluralityof outputs from the gated amplifiers 21 must occur before actuation ofthis relay occurs so as to ensure against its being operated throughrandom spurious outputs.

Channels 2 and 3 of Figs. 2B and 20, respectively, represent additionalchannels which may be provided as desired. Each of these channels isorganized in a manner similar to that of channel 1. Each receives acommon input over wire 24 from the differentiator-amplifier 19. In otherwords, each channel receives an output pulse each time that theoscillator frequency sweeps over the resonant frequency of a controlcoil which is inductively coupled to the receiving coil, regardless ofthe resonant frequency of such coil. At the same time, each channelreceives an input from the sweep frequency oscillator 17. The tunedcircuit elements which are included in the L-C resonant unit anddetector 22 provided for each channel provide a distinctive output onlyas the oscillator frequency sweeps over the range of frequenciesadjacent the particular resonant frequency for that channel. Thus, thecontemporaneous occurrence of an output from the L-C resonant unit anddetector for a particular channel with the occurrence of a ositive pulsefrom the diiferentiatoramplifier 19 is an indication that there is thencoupled with the wayside receiving coil, a vehicle-carried control coilwhich is resonant to the frequency associated with that channel.

The diiferent resonance frequencies of the various control coils must beproperly separated so that any control coil will be effective only withrespect to its associated channel and not with any of the otherchannels. The number of different control coil frequencies that may beused in a system is, therefore, afiected by the width of the frequencyband that the sweep frequency oscillator 17 can be made to operate over.In one embodiment of the invention, the sweep frequency oscillator wasorganized to operate over a range of frequencies from 150 kc., to 300kc. with four different control coil resonance frequencies included inthis frequency range. Also, in this embodiment of the invention a sweeprate of 300 cycles per second was selected, thereby ensuring that thesweep frequency oscillator would sweep over its frequency range severaltimes during the interval that control and receiving coils wereinductively coupled together for even the highest train speedscontemplated. These specific values are mentioned here only for purposesof illustration; other values can equally well be used in practice.

As previously mentioned, the inductive control systemthe vehicle, whilethe resonated control coils are then positioned at selected locationsalong the trackway.

When the inductive control system of this invention is to be used fortrain description purposes, the number of different train descriptionsinvolved may exceed the possi' ble number of different control coilresonance frequencies. In that event, it may 'be desirable to mount uponeach vehicle a plurality of control coils, each resonant to a differentone of selected resonance frequencies in either the manner shown in Fig.5A or 5B. In this manner, the combination of different resonancefrequencies determines the train description, and with a specifiednumber of different resonance frequencies available, the number ofdifferent train descriptions is appreciably in-- creased by this method.

When more than one control coil is thus to be used at a single location,it may be desirable to modify the receiving equipment by adding theapparatus shown in Fig. 4 within the dotted line block titled pulsestretcher driver 13!). The reason for doing so under these circumstancesis that a plurality of outputs is produced by thedifferentiator-amplifier 19 for each sweep of the oscillator frequencywhen more than one control coil is used at a location in the mannershown in Fig. 5B, one output being supplied for each control coil. Itwill readily be understood from the description of the pulsestretcheramplifier 20 previously given that its operation is dependentupon the reception of inputs from the differentiator-amplifier 19 at arate of one for each frequency sweep, with this rate determined by theblocking oscillator 13. Consequently, if more than one input is appliedto the pulse stretcher-amplifier for each frequency sweep a symmetrical,rectangular waveform is not produced.

The pulse stretcher driver shown in Fig. 4 has applied to it over wire24, the output obtained across the cathode resistor 87 of tube 86included in the differentiator-amplifier 19 which is shown in detail inFig. 2A. The output of the pulse stretcher driver 130 is applied throughcapacitor 89 to the control grid of tube 93 included in the pulsestretcher-amplifier 20. The pulse stretcher driver 130 includes aone-shot multivibrator comprising the triode tubes 131 and 132, adifferentiating amplifier including the tube 133, and

.cathode follower tube 134.

The circuit organization of a one-shot multivibrator is well-known inthe art so that a detailed description of its manner of operation willnot be presented. Briefly, however, tube 132 is in a normally conductivecondition because its control grid is connected through resistor 135 to(B+). A relatively low voltage thus appears at the plate of this tube132 because of the high voltage drop across plate resistor 136. Becauseof the presence of the biasing battery 137 in the grid circuit of tube131 and also because of the low plate voltage of tube 132, the gridvoltage for tube 131 is sufliciently low to cause this tube to benonconductive. A high plate voltage therefore results for the tube 131because there is then no voltage drop across the plate resistor 138.Capacitor 139 connected between the plate of tube 131 and the controlgrid of tube 132 is thus charged to a high potential.

As described, when more than one control coil is used at a singlelocation, a plurality of outputs is obtained from thedifrerentiator-amplifier 19 for each frequency sweep of the oscillator17. Each of these outputs from the differentiator-amplifier 19 appearsas a positive-going pulse on the wire 24 and is applied through thecapacitor 140 and decoupling resistor 141 to the control grid of tube131. The first-occurring of these positive pulses raises thegrid-cathode voltage of tube 131 above the cutoif level so that thistube begins to conduct plate cur- 15 voltage of tube 132 then tends toovercome the negative voltage provided by battery 137 so that the gridvoltage of tube 131 is held above cutoff.

At the same time, capacitor 139 begins to discharge through theconducting tube 131 with its rate of discharge determined primarily byits value of capacitance and the resistance of resistor 135. Ascapacitor 139 continues to discharge, the grid voltage of tube 132 risesuntil finally the grid-cathode voltage for this tube reaches cutoff. Atthat time, the tube 132 becomes conductive once more and the resultinglow plate voltage of this tube then causes tube 131 to again becomenonconducfive.

The time constant for the discharge of capacitor 139 is selected so asto keep tube 131 conductive for a time interval that is slightly lessthan the sweep repetition rate of the oscillator 17. Thus, themultivibrator can respond to the first-occurring output pulse from thedifferentiator-amplifier 19 but then cannot respond again until the nextfrequency sweep of the oscillator 17 results in an output produced bythe diiferentiator-amplifier 19 in response to the same control coilthat produced the response in the previous cycle.

As an example, if control coils resonant to the frequencies F1 and F2are employed at a single location, a separate output will be produced bythe diiferentiatoramplifier 19 for each of these control coils on eachfrequency sweep of the oscillator 17. If the first output from thediiferentiator-amplifier 19 results from inductive coupling of thereceiving coil with the control coil tuned to frequency F2, then themultivi'brator will respond to such output by causing tube 131 to becomeconductive and tube 132 nonconductive. Before the complete cycle time ofthe sweep oscillator has elapsed, an output from theditferentiator-amplifier 19 will be produced in response to the controlcoil tuned to frequency F1. At the time of occurrence of this output,however, tube 131 will still be conductive so that this pulse will havesubstantially no effect. The time constant for the discharge ofcapacitor 139 is so selected, however, that tube 131 will again becomenonconductive just prior to the second occurrence of an output from thedilferentiatoramplifier 19 in response to the control coil tuned tofrequency F2. As a result, tube 131 becomes conductive only once foreach frequency sweep of the oscilla tor 17, regardless'of how manycontrol coils are used at a location.

Each time that tube 131 becomes conductive, the normally chargedcapacitor 142 which connects the plate 'of tube 131 to the control gridof tube 133 is rapidly discharged. The time constant for the dischargeof capacitor 142 through resistor 143 is chosen to producedifferentiation of the plate voltage of tube 131 so that a sharpnegative pulse appears on the control grid of tube 133 each time thattube 131 becomes conductive. Whenever tube 131 becomes nonconductive,the increase of voltage that ends to appear on the control grid of tube133 merely causes grid current to flow so that the grid voltage does notrise appreciably.

Each negative pulse at the grid of tube 133 produces a correspondingnegative pulse at theplate'of'this'tube because of the momentarilyreduced plate current through plate resistor 134. The resultingpositivepulse appearing at the grid of cathode follower tube 144 producesa'similnr positive pulse across-the cathode resistor 145 so that apositive resistor is applied to the grid of tube 93 included in thepulse stretcher-amplifier 24).

By including the pulse stretcher-driver 130 in the receiving apparatus,it is-thus possible to supply to the pulse stretcher-amplifier 2t) aninput which comprises only a single pulse for-each sweep of the sweeposcillator 17. -The pulse strctehenamplifier '20 can thus respond, asal- 1 Having described aninductive control system as one embodiment ofthis invention, it should be understood that this form is selected tofacilitate in the disclosure of the invention rather than to limit thenumber of forms which it may assume. Also, various modifications,adaptations, and alterations may be applied to the specific form shownto meet the requirements of practice without in any manner departingfrom the spirit or scope of the present invention.

'WhatI claim is:

l. A system for transmitting controls at selected locations inductivelybetween a moving vehicle and fixed wayside locations along the trackwaythrough inductive cooporation between control coils at a controltransmitting location and a receiving coil associated with receivingapparatus, each of said control coils being resonated by respectivelyassociated capacitance to a selected frequency, said receiving coilpassing through an inductive coupling relationship with each of saidcontrol coils during train movement, said receiving apparatus includingan electron tube oscillator for energizing said receiving coil, tuningapparatus including a sawtooth generator for continuously-varying theoutput frequency of said oscillator over a range of frequenciesincluding said selected resonant frequencies of said control coils, saidtuning apparatus and said electron tube oscillator cooperating to varythe frequency of said oscillator at a repeti- 'tion rateto cause saidoscillator frequency to sweep over said range of frequenciesa-plurality-of times during the interval said control and receivingcoils are inductively coupledat maximum train-speeds, circuitmeansgoverned by said oscillatorand effective to provide a distinctive outputin responseto the loading effect produced on said oscillator when theoutput frequency of said oscillator sweeps over the resonant frequencyof a control coil coupled inductively to said receiving coil, acontrolled device, and means associated with said controlled device topermit actuation of said controlled device only in response to aplurality of successive occurrences of said distinctive outputs toprevent'spurious outputs.

2. A system for inductively transmitting controls between moving trainsand the trackway at fixed locations comprising,controlcoils at a controltransmitting location each being resonated to a selected frequency, asweep frequency oscillator including a receiving coil at a controlreceiving location, said sweep oscillator energizing said receiving coilwith an output frequency continuously sweeping between selected limitsat a fixed repetition rate, said receiving coil becoming inductivelycoupled With each control coil at said control transmitting locationduring train movement, reaction detecting means governed by saidoscillator and effective to provide a distinctive output in response tothe loading of said sweep frequency oscillator by said control coil, acontrolled device, circuit means responsive only to successiveoccurrences of said distinctive outputs for supplying a steady currentto said controlled device, means associated with said controlled deviceand effective to cause actuation of said controlled device inresponse tosaid steady current only provided that said steady current persists fora selected time interval.

3. A system for transmitting controls at selected locations between amoving vehicle and the trackway through inductive cooperation betweencontrol coils at a control transmitting location and areceiving coilassociated with receiving apparatus, each of said control coils beingresonated to aselected frequency by respectively associated capacitance,an electron tube oscillator for energizing said receiving coil,electronic tuning apparatus for continuously varying the outputfrequency of said oscillator over a range of frequencies which includessaid selected resonant frequency of said control coil, reactiondetecting circuit means associated with said oscillator and effective toproduce a distinctive output in response to the termination of theloading effect produced on said oscillator when the output frequency ofsaid oscillator sweeps over the resonant frequency of a control coilinductively coupled to said receiving coil, said tuning means eifectiveto vary the frequency of said oscillator over said range in the samedirection on successive frequency sweeps with an abrupt return betweensuccessive sweeps-to the starting frequency, resonant circuit meansincluded in said receiving apparatus tuned to the resonant frequency ofsaid control coil and having the output of said oscillator appliedthereto, a controlled device, means responsive to the contempt raneousoccurrence of said distinctive output and the increase of voltageoccurring across said resonant circuit to actuate said controlleddevice, the unidirectional frequency sweep of said oscillator causingsuccessive of said distinctive outputs to occur at substantially thesame frequency of said oscillator, said resonant circuit means beingtuned to resonate over a relatively narrow frequency band encompassingsaid same frequency of said oscillator, to thereby render ineffectivespurious outputs from said reaction detecting circuit means occurring atother frequencies of said oscillator.

4. A system for transmitting controls at selected locations between amoving vehicle and the trackway through inductive cooperation betweencontrol coils at a control transmitting location and a receiving coilassociated with receiving apparatus, said control coils at each locationresonated by respectively associated capacitance to different selectedfrequencies, an electron tube oscillator for energizing said receivingcoil, tuning means for continuously rapidly varying the output frequencyof said oscillator over a selected frequency range including saidselected resonant frequencies of said control coils, said tuning meansbeing effective to vary said oscillator frequency at a repetition rateeffective to cause said oscillator frequency to vary over said frequencyrange a plurality of times during the interval said control coils andsaid receiving coils are inductively coupled even at maximum trainspeeds, reaction detecting circuit means associated with said oscillatorand effective to produce a distinctive output in response to eachloading of said oscillator occurring as the output frequency of saidoscillator sweeps over the resonant frequency of a control coilinductively coupled to said receiving coil, a master channel comprisinga controlled device and circuit means for causing actuation of saidcontrolled device only in response to a preselected plurality ofoccurrences of said distinctive outputs, an individual frequency channelassociated with each of said control coil resonant frequencies, sweepposition determining circuit means included in said individual channeleffective to produce a distinctive gating voltage as the outputfrequency of said oscillator sweeps over said resonant frequencycorresponding to said individual channel, gated circuit means includedin said individual channel responsive to the contemporaneous occurrenceof said distinctive gating voltage and said distinctive output from saidreaction detecting circuit means and effective to produce an outputvoltage pulse, a controlled device included in said individual channel,circuit means governed by said voltage pulses and effective to causeactuation of said controlled device only in response to a preselectedplurality of occurrences of said voltage pulses, circuit meansresponsive to the contemporaneous actuation of said controlled deviceassociated with said master channel and said controlled device of saidindividual channel to designate a particular control.

5. A system for inductively transmitting controls between moving trainsand the trackway at fixed locations comprising, control coils at acontrol transmitting location each being resonated to a selectedfrequency by respectively associated capacitance, a sweep frequencyoscillator associated with receiving apparatus and including a receivingcoil, said sweep oscillator energizing said receiving coil with anoutput frequency range continuously sweeping between selected limits ata fixed repetition rate, said receiving coil becoming inductivelycoupled with each control coil during train movement, reaction detectingcircuit means effective to produce a distinctive output in response tothe loading effect on said oscillator occurring when said oscillatorfrequency sweeps over said frequency of each control coil during thetime said control coil is inductively coupled to said receiving coil,tuned circuit means resonated to said resonant frequency of said controlcoil, circuit means for applying the output of said sweep frequencyoscillator to said tuned circuit means, two electron discharge tubeshaving a common plate load resistor, circuit means responsive to theenvelope of oscillator voltage appearing across said tuned circuit meansfor controlling one of said tubes to a nonconductive condition, circuitmeans responsive to the occurrence of said distinctive output from saidreaction detection means for controlling the other of said tubes to anonconductive condition, a controlled device, circuit means responsiveto the high voltage appearing at the plates of said tubes when saidtubes are simultaneously controlled to nonconductive conditions toactuate said con trolled device.

6. A system for inductively transmitting controls between moving trainsand the trackway at fixed locations comprising, control coils at acontrol transmitting location each resonated by respectively associatedcapacitance to a selected frequency, a sweep frequency oscillatorassociated with receiving apparatus, a receiving coil included in thecircuit organization of said sweep frequency oscillator, said sweepfrequency continuously varying between selected upper and lowerfrequency limits at a fixed repetition rate, said receiving coilbecoming inductively coupled with each control coil during trainmovement, reaction detecting means associated with said oscillator andeffective to provide a distinctive positive voltage pulse in response toeach inductive loading of said oscillator by a resonated control coil,circuit means responsive to the repetitive output pulses from saidreaction detecting means and comprising a grid controlled electrondischarge tube, said grid being connected to a positive voltage sourcethrough a grid resistor, circuit means for applying said positive pulsesto said grid through a coupling capacitor, said capacitor having adischarge time through said grid resistor effective to maintain saidtube nonconductivc following the occurrence of each of said distinctivepulses for an interval equal substantially to one-half the sweeprepetition rate of said oscillator, an electromagnetic relay, rectifyingcircuit means responsive to the plate current of said tube forenergizing said relay, a capacitor shunting the winding of said relay,whereby a plurality of distinctive outputs must be produced by saidreaction detector circuit means to cause the energization of said relayto persist for a long enough time interval to actuate said relay.

7. In a system for inductively transmitting controls at selectedlocations between a moving vehicle and the trackway through inductivecooperation between control coils at a control transmitting location anda receiving coil associated with receiving apparatus, each of saidcontrol coils being resonated to a selected frequency by an associatedcapacitance, said receiving equipment including an electron dischargetube, a saturable transformer having primary and secondary windings, aplate-cathode circuit for said tube including said primary winding, agridcathode circuit for said tube, circuit means for applying to saidgrid-cathode circuits a voltage varying over a selected range at apredetermined repetition rate, said voltage initially varying rapidlyfor a short time interval and then varying more slowly in the samedirection for the remainder of said interval, the level of said voltageat the end of said voltage variation returning abruptly to a fixedstarting level, an electron tube oscillator, a tank circuit for saidoscillator including said receiving coil and said secondary winding,whereby said voltage variation causes said oscillator frequency to varysubstantially linearly over a selected frequency range including saidresonance frequencies of said control coils.

8. A system for transmitting controls at selected 10- cations betweenmoving vehicles and the trackway through the inductive cooperationbetween control coils at a control transmitting location and a receivingcoil associated with receiving apparatus, a plurality of control coilsat each control transmitting location, each of said control coils at alocation being resonated by an associated capacitance to a differentfrequency, a sweep frequency oscillator for energizing said receivingcoil, said oscillator output frequency continuously sweeping over afrequency range including said resonance frequencies of said controlcoils at a predetermined repetition rate, reaction detecting meanseffective to produce distinctive voltages in response to the loadingeffect upon said oscillator produced as said output frequency of saidoscillator sweeps over said resonance frequencies of said control coils,a master channel for said receiving apparatus including first circuitmeans responsive to said distinctive voltages, said circuit means beingrendered ineffective in response to an occurrence of one of said.distinctive outputs to respond to successive occurrences of saiddistinctive outputs for a time interval just less than thetirne requiredfor said oscillator to sweep overits .entire frequency range, acontrolled device, second circuit means governed by the output of saidfirst circuit means for energizing said controlled device, meansassociated with said controlled device to permit said controlled deviceto be actuated only in response to a plurality of successiveoccurrencies of said distinctive outputs.

9. A system for transmitting controls at selected locations between amoving vehicle and the trackway through the inductive cooperationbetween control coils at a control transmitting location and a receivingcoil associated with receiving apparatus, a plurality of control coilseach being resonated to a different frequency by an associatedcapacitance at each of said control transmitting locations, a sweepfrequency oscillator for energizing said receiving coil, said oscillatoroutput frequency continuously sweeping over a frequency range includingsaid resonance frequencies of said control coils at a predeterminedrepetition rate, reaction detecting means effective to producedistinctive voltages in response to the loading effect upon saidoscillator produced as said output frequency sweeps over said resonancefrequencies of each of said control coils, a master channel for saidreceiving apparatus including a one-shot multivibrator, circuit meansfor controlling said multivibrator from its normal condition in responseto said distinctive voltages, said multivibrator controlled to return toits normal condition after a time interval just less than the timeinterval of a complete frequency sweep by said sweep frequencyoscillator, circuit means governed by said multivibrator to give adistinctive output when said multivibrator is controlled from its normalcondition, a controlled device, means responsive to said distinctiveoutputs for governing said controlled device, means associated with saidcontrolled device to permit said controlled device to be actuated onlyin response to a plurality of successive occurrences of said distinctiveoutputs.

References Cited in the file of this patent UNITED STATES PATENTS2,150,857 Edwards Mar. 14, 1939 2,291,715 Hepp Aug. 4, 1942 2,407,270Harrison Sept. 10, 1946 2,407,644 Benioif Sept. 17, 1946 2,416,320Jeanne Feb. 25, 1947 2,419,340 Easton Apr. 22, 1947 2,454,687 BaughmanNov. 23, 1948 2,488,815 Hailes Nov. 22, 1949 2,509,632 Field May 30,1950 2,510,066 Busignies June 6, 1950 2,512,305 Clapp June 20, 19502,535,162 Rodgers Dec. 26, 1950 2,562,295 Chance July 31, 1951 2,596,167Philpott May 13, 1952 2,597,517 Noble May 20, 1952 2,673,292 TreharneMar. 23, 1954 2,693,525 Kendall et al Nov. 2, 1954

